Add missing image upload properties

src/test/resources/arsnova.properties.example

@@ -194,6 +194,10 @@ feedback.cleanup=10
 # maximal filesize in bytes
 upload.filesize_b=1048576
 
+# maximal thumbnail size in pixels
+imageupload.thumbnail.width=200
+imageupload.thumbnail.height=200
+
 # maximal number of answer options allowed for a skill question
 question.answer-option-limit=8